1. Field of Invention
This invention relates generally to electromagnetic flowmeters, and more particularly to a flow meter which is operated from a standard alternating current power line and whose electromagnetic field is produced by pole pieces of "hard" magnetic material that are alternately reversed in magnetization by coils associated therewith to which driver pulses are applied that are derived from the line voltage in a manner requiring relatively little power.
2. The Prior Art
In an electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by electromagnets. When the flowing liquid intersects the field, a voltage is induced therein which is transferred to the electrodes to provide an output signal. This signal is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications.
The adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induced unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as for example, with continuous d-c operation, the ideal condition d.phi./dt=0 is satisfied. But, as previously noted, d-c operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
In the patent to Mannherz et al., U.S. Pat. No. 3,783,687, and in my prior U.S. Pat. No. 4,370,892, whose entire disclosures are incorporated herein by reference, there are disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects. This low frequency wave is derived by means of a presettable scaler coupled to the standard a-c power line (60 Hz) and is at a frequency in the order of 17/8, 33/4, 71/2 or 15 Hz.
In conventional electromagnetic flowmeters, the electromagnetic field is established by a pair of electromagnets placed at diametrically-opposed positions on the tube. Each electromagnet is constituted by a core of "soft" magnetic material having a coil wound thereon which, when supplied with current, acts to strongly magnetize the core. When current flow is interrupted, the core is then almost completely demagnetized.
Commercial magnetic materials are divided into two main groups: (1) magnetically "soft" materials, and (2) magnetically "hard" materials. The distinguishing characteristic of "soft" magnetic materials is high permeability. These materials are usually employed as cores or the magnetic circuits of electromagnets. "Hard" magnetic materials are characterized by a high maximum magnetic energy product (BH).sub.max. These materials are employed as permanent magnets to provide a constant magnetic field when it is inconvenient or uneconomical to produce this field by an electromagnet.
The chief drawback of the conventional electromagnetic flowmeter which uses soft magnetic materials is that its power requirements are substantial. In order to reduce these power requirements, the U.S. Pat. No. 4,409,846 to Ueno discloses a meter which makes use of hard magnetic materials in association with coils to which pulses of alternating polarity are applied to periodically demagnetize the magnets. While the arrangement disclosed in this patent does bring about a reduction in power consumption, it is not adapted to be operated from a standard alternating-current power line and to take advantage of the frequency of the line current to produce alternating excitation pulses for the coils.